1. Field of the invention
This invention relates to heavy duty pneumatic tires. More particularly it relates to a heavy duty pneumatic radial tire capable of advantageously attaining reduction of belt weight.
2. Description of the Related Art
Recently, it is strongly demanded to improve a fuel consumption in an automobile from viewpoints of resource-saving and energy-saving. It is known to reduce tire weight to achieve the improvement of the fuel consumption. As to reducing tire weight, it is effective to reduce the weight of each member constituting the tire. Among the members, the weight reduction of a belt is particularly effective because a ratio of the belt containing rubberized steel cords in the total weight of the tire is large.
The belt reinforcing this type of the tire is comprised of 3-4 belt layers and it is common to have a first belt layer containing cords inclined at a large cord angle with respect to an equatorial plane of the tire and at least two belt layers each arranged outside the first belt layer in a radial direction of the tire and containing cords inclined at a small cord angle with respect to the equatorial plane, in which cords of a second belt layer and a third belt layer are crossed with each other. As the cord constituting each belt layer, there are adopted 3+6 structure, 1.times.12 structure, 3+9+15+1 structure and the like.
As a countermeasure for reducting the weight of the belt, it is effective to decrease the end count of cords in the belt layer, or to increase the tensile strength of the steel material to decrease the quantity of steel used while maintaining the total tenacity of the belt layer.
In these countermeasures, however, the volume ratio of cords in rubber of the belt layer decreases to lower the tensile rigidity of the belt. As a result, the outer diameter of the tire is largely increased at the time of inflation under an internal pressure or due to creep phenomenon during the use of the tire to bring about the occurrence of separation failure at the belt end or promote uneven wear. Furthermore, the lowering of the tensile rigidity in the belt brings about the decrease of cornering power and the degradation of wear resistance.